Conventionally, apparatuses which pick up an image of a three-dimensional object by irradiating the object with light and inspect an appearance of the object on the basis of the picked-up image have been used. In an appearance inspection apparatus disclosed in Japanese Patent Application Laid-Open No. 2005-17234, for example, in order to inspect an appearance of a dome-shaped solder on an electronic circuit board, a first image is acquired in a state where the dome-shaped solder is irradiated with parallel rays of light from both left and right sides thereof and a second image is acquired in a state where the dome-shaped solder is irradiated with parallel rays of light from front and back sides thereof. Then, a composite image which is represented by absolute values of differences between first image data and second image data is obtained, and when strip-like shades are present radially on the composite image, a mounting failure of a chip component on the dome-shaped solder is detected.
Further, in a shape recognition apparatus disclosed in Japanese Patent Application Laid-Open No. 2009-162573, a camera for picking up images of an object to be inspected and a lighting part rotating about the camera are provided, and images of the object to be inspected are sequentially picked up with lighting angles of the lighting part being changed. In the shape recognition apparatus, since a shadow of a projection (defective shape) on the object to be inspected is changed as the lighting angle is changed, the shape of the projection can be estimated.
On the other hand, in a metal component (e.g., automobile component) formed by forging and/or casting, a surface treatment such as shot blasting and/or the like is performed thereon and a surface of the component thereby has a satin-finished three-dimensional structure with microscopic projections and depressions thereon. In an appearance inspection on such metal components, defects such as dents, flaws, and the like on surfaces of the objects are detected by visual inspection of operators.
In such an inspection of the objects by operators as above, even if an inspection standard is determined, there are disadvantageously variations in the accuracy of the inspection among the operators. Further, there is a possibility that defects of the objects may be overlooked by human error. In the case where defects are detected on the basis of the picked-up images of the objects, since the light entering the satin-finished surfaces is reflected diffusely, variations in the tone value of the picked-up image (local changes in the density) increase and therefore a number of false defects are disadvantageously detected.